1. General Description of the Field of the Invention
The general principles underlying the construction and operation of prior art flux lock loops are taught in various printed publications. In particular, reference is made to an article entitled "Lock-On Magnetometer Utilizing a Superconducting Sensor", by R.L. Forgacs, which appeared in the September, 1966 issue of IEEE Transactions of Instrumentation and Measurement. The flux locked loop particularly described therein employs a squid (superconducting quantum interference device) having two weak links, and is embodied in an instrument for measuring magnetic fields.
The squid used in a flux locked loop need not be of the type having two weak links. In this connection, reference is made to U.S. Pat. No. 3,622,881 to Silver et al, which shows a flux locked loop employing a squid with a single weak link. This patent also shows that it is known to embody a flux locked loop in an instrument other than a magnetometer. In particular, Silver et al describe instruments such as voltmeters and voltage generators embodying flux locked loops.
In a flux locked loop, a squid defines an aperture through which a time-varying amount of flux is threaded. The squid is biased such that it exhibits a periodic transfer characteristic. Different types of biasing techniques are used for different types of squids.
For a squid having two weak links, a direct current biasing technique is typically used in which first and second pairs of wires are electrically connected to the squid for carrying bias current and for carrying a squid output signal respectively. With this technique, the biasing current flowing through the first pair of wires is direct current and this direct current flows through the squid with the two weak links being in parallel paths with respect to the current flow. For a super-conducting region of operation (i.e., while device current is less than the critical current), the voltage appearing between the two wires of the second pair, which are connected across the parallel weak lines, is null. For a quenched region of operation (i.e., while device current is greater than the critical current), a voltage drop appears across the parallel weak links. In short, the squid output voltage appearing across the parallel weak links constitutes a critical-current-representing signal. Critical current is a periodic function of the amount of flux threading the squid aperture. Accordingly, under properly biased conditions, the double-weak-link squid exhibits a periodic transfer characteristic. Thus, where the time-varying amount of flux increases continuously, for example, the squid output voltage signal varies back and forth between maximum and minimum values, with a periodicity determined by a universal constant (h/2e, where h is Planck's constant and e is electronic charge).
For a squid having a single weak link, an alternating current biasing technique is typically used. In this technique, a relatively high frequency carrier signal is applied to a tuned circuit inductively coupled to the squid so as to expose the squid to an oscillatory biasing flux. As a result, a screening current is induced in the squid whereby the device critical current can be cyclically exceeded. Under properly biased conditions, the single-weak-link squid and associated circuitry (such as the tuned circuit and a demodulator for the carrier signal) exhibit the same kind of periodic transfer characteristic as described above, and operate to provide a critical-current-representing signal.
From what has so far been described, it will be appreciated that, with either type of squid, the flux cannot be uniquely or unambiguously determined from the critical-current-representing signal itself. That is, the critical-current-representing signal has the same value for a plurality of different discrete values of flux. Moreover, the slope of the transfer characteristic, unlike the periodicity thereof, is not determined by a universal constant and varies from device to device. It is for these reasons that feedback is employed in a flux locked loop.
As shown by the above-referenced Forgacs' article, one arrangement employing feedback includes a feedforward path and a feedback path. A field coil in the feedback path receives a feedback current, and, the flux it thereby produces affects the amount of flux threading the squid aperture. In addition to receiving the feedback current, the field coil also receives the output of an oscillator. As a result of this arrangement, an electrical signal having a modulation frequency component is produced in the feedforward path. A synchronous detector demodulates this electrical signal to produce an error signal which is applied to an integrator forming the output stage of the feedforward path. Owing to the memory provided by the integrator, the error signal can be nulled and yet the integrator output signal can be representative of the amount of flux being measured.
2. Problems Incurred With Prior Art Flux Locked Loops
One of the problems incurred with prior art flux locked loops relates to the effects of ground-loop noise disturbances. In this connection, it should be understood that the squid and any coil coupled to it are disposed in a cryostat and cooled to a superconducting-temperature level of about 4.degree. Kelvin. It is preferable, of course, for most of the circuitry of the flux locked loop to operate in a room-temperature environment. It is necessary to use relatively thin wires to make the electrical connection between the coil and the circuitry. These wires, being exposed to a substantial temperature gradient, exhibit thermo electric voltages causing disturbance inputs. Moreover, this electrical connection in general involves connections between dissimilar metals. Again, this is a source of a thermo electric voltage disturbance input.
Separately, it is desirable to use a single coil lightly coupled to the squid to perform multiple functions. That is, a simpler mechanical structure is obtained where the same coil is used for radio frequency carrier excitation of the squid and for receiving feedback current. Where a single coil is used, it is necessary to electrically connect one terminal of the coil to the cryostat. The grounding of the coil to the cryostat and the spacing of the coil from the room-temperature circuitry are principal factors underlying ground-loop disturbances. In particular, ac and dc currents flow as a result of such ground loops with the undesirable result of noise voltages being developed in the feedback circuitry. In a prior art arrangement such as that disclosed in the Forgacs' article, where a simple series feedback resistor is used, the effect of the ground-loop disturbance can be expressed as i.sub.FB =[(Vo/R.sub.FB)+(Ee/R.sub.FB)]. In this equation, i.sub.FB represents the feedback current to the coil; Vo represents the analog output voltage of the feedforward path; Ee represents the Thevinen equivalent voltage of the ground-loop noise voltages; and R.sub.FB represents the feedback resistor. Ideally, i.sub.FB should equal Vo/R.sub.FB. Thus, the foregoing equation shows that i.sub.FB is in error by an amount Ee/R.sub.FB.
Another problem with prior art flux locked loops is as follows. The multi-valued, periodic transfer characteristic exhibited by the squid renders the flux locked loop susceptible to an undesirable shift from one lock point to another. That is, under given conditions, the operating point of the squid with respect to its transfer characteristic can be locked at a particular point within a first linear region thereof. Then, a disturbance input such an interference transient can cause circuitry within the feedforward path to develop a relatively large voltage. The integrator in the feedforward path can respond to this noise voltage whereby its output voltage swings sufficiently to cause the operating point of the squid to shift to a second linear region of the transfer characteristic. The result is a most undesirable permanent offset error.
Another problem with prior art flux locked loops is as follows. The multi-valued, periodic function relating the error signal to the average flux may be expressed in general form as: ##EQU1##
The foregoing equation defines a Fourier series with coefficients A.sub.n. The term u.sub.o represents the period of the transfer function. The symbol .phi..sub.o is often used in the literature to refer to the period of the transfer function. Hereinafter, .mu..sub.o and .phi..sub.o are used interchangeably. To obtain an optimum feedback system, a modulation technique is employed. That is, an oscillator operating at a modulation frequency w.sub.m and a coil responsive thereto cooperate to modulate the flux threading the squid aperture. This has the effect of heterodyning input signals to form sidebands of the modulation frequency. The heterodyning process is most efficient when the amplitude of the modulating wave is exactly .phi..sub.o /2 The prior art flux locked loops have not been arranged in a manner that facilitates adjusting the modulation to achieve this desirable condition.